The present invention pertains to a method for the production of spherical granulated materials from powdered solids and also to the granulated materials produced by this method.
In many industrial realms there is a problem of storing, transporting, and processing of powdered solids. This is often associated with a significant expense since the powdered substances have physical properties that differ from those of pure solids. The technical handling of such materials is improved if they are present in granulated material form. To do this, the powders are usually compressed mechanically with the addition of suitable binders to such an extent that interactive forces are in effect at the surfaces between the individual powder particles in the molecular range. In this case products form with new properties.
Spherical granulated materials are of particular interest. They have improved flow properties compared to finely dispersed bulk material and cylindrical extrudates or tablets; thus they can be metered in, transported and stored at lower cost. In many cases the granulated material solids have to be dispersed again during use, for example in the case of laundry detergents and when using carbon black as a structural enhancer in rubber articles (rubber blacks) or for dyeing of paints and plastics (pigment blacks).
The powdered carbon blacks occurring during production thereof must therefore be granulated in a subsequent process step. In this case, in addition to a small fraction of fines and a narrow particle size distribution, a simple and complete redispersibility of the granules is desirable in spite of their durability (transport stability).
The redispersibility plays an important role for example in applications in the area of paint production, cable compounds and technical rubber articles, since even small fractions of nondispersed carbon black agglomerates can lead to severe deficiencies in the application of the end product to its ultimate uses.
The term "carbon black agglomerates" as used herein means the agglomerates of a number of so-called carbon black aggregates bound together by physical cohesion forces (Van der Waals forces) according to the definitions in DIN (German Industrial Standard) 53206. Carbon black aggregates are produced by solid-phase coalescence of the primary carbon black particles during the carbon black formation process from which carbon black agglomerates form through coalescence of the particles either spontaneously or promoted by a granulation process.
Carbon blacks with a high fraction of voids in the aggregate are called "high structure" carbon blacks; in the case of a small percentage of voids, they are called "low structure" carbon blacks. The dibutyl phthalate absorption method (DBP absorption according to DIN 53601) is used as a measure for the structure of a carbon black.
The carbon black structure in large measure determines the product properties of the paints, plastics or rubber articles dyed or reinforced with it. Therefore, in order not to adversely affect the product properties, the smallest possible break up of the structure by the granulation process is required.
Granulated, powdered solids are also employed as carriers for chemical catalysts and as adsorbents. In this case, by means of the material itself or by addition of suitable binders, it must be assured that the granules do not break apart in use. Good flow properties and metering properties and also a low abrasion are also required here as well.
Various methods for granulation and/or agglomeration of powdered solids are known, for example from Ullmann's Encyclopedia of Industrial Chemistry, Vol. B2, pages 7-15 to 7-21. A distinction is made between spray agglomeration and/or spray drying, coalescence agglomeration and agglomeration by mechanical compacting (press agglomerates). The agglomerates produced in these ways have dimensions in the range of several tenths of a millimeter up to several millimeters, and for purposes of this patent application are designated as granulated materials in order to establish a definite differentiation from the carbon black agglomerates defined according to DIN 53206 that are produced by the spontaneous coalescence of carbon black aggregates and that have dimensions of less than 100 .mu.m.
The granulation of powdered solids is usually carried out with addition of a suitable binder and of a moistening agent which is normally water. The moistening agent must be removed down to a certain residual moisture level after the granulation by means of energy-intensive drying methods. Depending on the end application, a calcination step at elevated temperatures is also needed.
Typical applications of these agglomeration methods are the spray drying of laundry compounds and fodder, and the coalescence agglomeration of carbon black in barbed shaft mixers, plowshare or counter current mixers. The agglomerates and/or granulated materials produced in these ways are not uniformly compacted.
Due to the relatively broad distribution of particle sizes, the flow properties of the loose granulated materials pile is adversely affected. This impedes a precise metering during use. The addition of moistening agents and binders can have negative effects on the redispersibility of the granules.
Roller compactors, matrix presses and extruders are used in the case of agglomeration methods using mechanical compression. The granulated materials and/or agglomerates produced by these machines have a cylindrical shape and have to be rounded off in rounding units to form the spherical shape. Now it turns out that quite different granulated materials qualities are obtained. The diameter distribution of the granulated materials in general has a very broad range, and in addition to the desired spherical granules they also contain tiny fragments that have to be sifted off in an additional process step in order to obtain a dust-free product. The physical properties of these granules, such as homogeneity, fracture resistance, abrasion and redispersibility, exhibit large fluctuations and cannot be matched to each other satisfactorily by known methods.
Extruders produce very homogeneous and highly compressed agglomerate strands, but extruders require a very precise setting of the moisture and kneading time of the powdered solid. Typically the permissible tolerances for the moisture of the infed material is in the range of only 1 to 2%. The investment costs for extruders are high.
As mentioned, in many technical applications it is a matter of producing granulated materials with the best possible fracture strength and small abrasion with simultaneously good redispersibility. These requirements are mutually contradictory and cannot always be completely fulfilled by the known agglomeration and/or granulation methods.